ISIS Redoubt in Northern Syria Is Said to Be Nearly Encircled

An important northern Syrian city held by the Islamic State, the group’s last major outpost in the region, was practically surrounded Monday by pro-government and insurgent forces squeezing it from different directions, according to rebels and the state-run Syrian news media.

The city — Al Bab, in Aleppo Province, 15 miles south of the Turkish border — has been hemmed in for months by insurgent fighters and Turkish troops, who have together blocked approaches from the east, north and west, according to rebels working with Turkey, as well as local activists and the Syrian Observatory for Human Rights, a monitoring group.

But now the Syrian Army, aided by Hezbollah militia fighters and artillery units supplied by Russia, has cut off the approach to Al Bab from the south, the Syrian Observatory said Monday. The state-run news media corroborated its account, saying the pro-government forces had captured a hill overlooking a southeastern route out of Al Bab, the last road not already blocked by the Turkish troops and rebels.

Although the Turkish-backed insurgents and Russian-backed government forces are not openly coordinating their moves — and in other circumstances, might be attacking each other — the joint encirclement of Al Bab appeared to reflect a strategic desire to rout the Islamic State, a group that both parties describe as an enemy.

Turkey and Russia are also pressing ahead with diplomatic cooperation, with the goal of expanding a partial truce and bringing the Syrian antagonists into a new round of peace talks.

But as the antagonists get closer to Al Bab, the encirclement could become a flash point. If Islamic State fighters flee, pro- and antigovernment forces could find themselves facing each other across an unpredictable new front line.

Al Bab is one of the largest urban areas in central northern Syria, after Aleppo, the city retaken from rebel groups by Syrian forces, with Russian help, nearly two months ago. Since then, President Bashar al-Assad of Syria has focused more military resources on fighting the Islamic State, particularly in Palmyra, the ancient city that the group captured in May 2015, lost last March and recaptured in December.

If the Islamic State is defeated in Al Bab, it will lose its last piece of territory in Aleppo Province, where it once held large areas. That would winnow the group’s significant Syrian territory to areas around Raqqa, its self-declared capital farther east.

That possibility has elevated the importance of Al Bab to all sides in the conflict, which is nearly six years old.

The Syrian Army’s occupation of the southern approach to Al Bab gives it control of “the only and last main road used by the jihadists between Al Bab and Raqqa,” said the Syrian Observatory, which bases its reporting on a network of contacts on the ground.

Turkey and Russia are working to advance peace talks on Syria, although they support opposite sides. A brief round of talks held last month in Astana, Kazakhstan, is set to resume this month in Geneva. The Islamic State and extremist militant factions tied to Al Qaeda are excluded.

The Islamic State is increasingly under assault in Syria and Iraq, where an American-backed military effort to evict the group from the northern city of Mosul has been underway for months. Roughly half the city has been retaken.

In a report issued on Monday, the United Nations said the Islamic State had been further constrained by declines in revenue from its oil-smuggling and extortion businesses, as well as fewer recruits.

The report said the group was “militarily on the defensive in several regions, notably in Afghanistan, Iraq, Libya and the Syrian Arab Republic.”

Nonetheless, it cautioned, the Islamic State has partly adapted and “continues to encourage its followers and sympathizers outside conflict zones to perpetrate attacks.”

Anne Barnard and Somini Sengupta contributed reporting.

A version of this article appears in print on , Section A, Page 6 of the New York edition with the headline: ISIS Redoubt in Syria Is Said to Be Nearly Encircled. Order Reprints | Today’s Paper | Subscribe